1. Field of the Invention
This invention relates to patterned media on hard disk drives and more particularly relates to variable clock rates for patterned media within a sector.
2. Description of the Related Art
Hard-disk drives provide data storage for data processing systems in computers and servers, and are becoming increasingly pervasive in media players, digital recorders, and other personal devices. Advances in hard-disk drive technology have made it possible for a user to store an immense amount of digital information on an increasingly small disk, and to selectively retrieve and alter portions of such information almost instantaneously. Particularly, recent developments have simplified hard-disk drive manufacture while yielding increased track densities, thus promoting increased data storage capabilities at reduced costs.
In a hard-disk drive, rotating high precision aluminum or glass disks are coated on both sides with a special thin film media designed to store information in the form of magnetic patterns. Electromagnetic read/write heads suspended or floating only fractions of micro inches above the disk are used to either record information onto the thin film media, or read information from it.
A read/write head may write information to the disk by creating an electromagnetic field to orient a cluster of magnetic grains in one direction or the other. Each grain will be a magnetic dipole pointing in a certain direction and also creating a magnetic field around the grain. All of the grains in a magnetic region typically point in the same direction so that the magnetic region as a whole has an associated magnetic field. The read/write head writes regions of + and − magnetic polarity, and the timing of the boundaries between regions of opposite polarity (referred to as “magnetic transitions”) is used to encode the data. To increase the capacity of disk drives, manufacturers are continually striving to reduce the size of the grains.
The ability of individual magnetic grains to be magnetized in one direction or the other, however, poses problems where grains are extremely small. The superparamagnetic effect results when the product of a grain's volume (V) and its anisotropy energy (Ku) fall below a certain value such that the magnetization of that grain may flip spontaneously due to thermal excitations. Where this occurs, data stored on the disk is corrupted. Thus, while it is desirable to make smaller grains to support higher density recording with less noise, grain miniaturization is inherently limited by the superparamagnetic effect.
In response to this problem, engineers have developed patterned media, where the magnetic thin film layer is created as an ordered array of highly uniform islands, each island capable of storing an individual bit. Each bit may be one grain, or several exchange coupled grains, rather than a collection of random decoupled grains. In this manner, patterned media effectively reduces noise by imposing sharp magnetic transitions at well-defined pre-patterned positions, known as bit patterns. Bit patterns are organized as concentric data tracks around a disk.
A head-positioning servomechanism facilitates the ability of a read/write head to locate a particular data track location and to reposition the head from one location to another. Indexing marks and alignment indices may be recorded in arc-shaped regions of the disk surface, known as servo sectors, and referenced by the servomechanism to maintain proper dynamic positioning capabilities of the read/write head over time. Track addresses, synchronization signals, and position error signal bursts may also be recorded in servo sectors.
Disk drives using patterned media require synchronization of a write clock to the pre-patterned positions of islands along a track on the disk during writing. This synchronization is critical to ensure that the write head writes data when the write head is over an island. If the write head attempts to write data in between islands, the data will be lost. A problem arises with patterned media in that the write clock becomes out of synch with the islands over the course of normal operation.
One solution to this problem is a method called “sector synchronization.” In a sector synchronization system the disk drive reads a once-per-sector synchronization feature at the beginning of each sector, locks the write clock to the correct frequency and phase, and proceeds to write data with this frequency and phase for the remainder of the sector. Unfortunately, any error in frequency due to noise in the synchronization process, or any speed variation in the disk can cause a write clock error.
The magnitude of such errors will increase as the write head proceeds from the beginning of the sector towards the end of the sector. Resynching at the beginning of the next sector eliminates this growing error. In order to compensate for the write clock becoming out of sync with the islands, the placement and spacing of the islands is carefully considered.
However, this margin of error limits the data density possible with patterned media. From the foregoing discussion, it should be apparent that a need exists for a method and apparatus with the ability to vary the clock rate of the write clock within a sector.